Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0426—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
  • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• the present invention relates to a high tensile strength galvanized steel sheet having tensile strength (TS) of at least 980 MPa and excellent formability, which is suitable for a material of automobile parts and the like.
• TS tensile strength
• the present invention also relates to a method for manufacturing the high tensile strength galvanized steel sheet.
• a hot rolled steel sheet as a material of a chassis member in particular is required to have good strength, formability and corrosion resistance as important characteristics thereof and there is a demand in this regard for a high tensile strength hot rolled (galvanized) steel sheet being excellent in formability such as elongation and stretch-flange ability because a chassis member in general has a complicated shape and is exposed to a harsh corrosion environment.
• a steel sheet as a material of a skeleton member of an automobile body is further required to have excellent bending properties regarding formability thereof.
• JP-B 3591502 proposes as a technique of increasing strength of a steel sheet, while ensuring good formability thereof, a technique regarding a high tensile strength steel sheet having tensile strength of >590 MPa and excellent formability, characterized in that: microstructure of the steel sheet is substantially constituted of ferrite single phase; and carbides including Ti and Mo having the average particle diameter of 10 nm or less are dispersion-precipitated therein.
• the technique of JP-B 3591502 however, has a problem of significantly high production cost due to use of expensive molybdenum.
• JP-A 2006-161112 proposes a high-strength hot rolled steel sheet having ⁇ 880 MPa tensile strength, ⁇ 0.80 yield ratio, a microstructure which contains ⁇ 70 vol.% of ferrite having ⁇ 5 ⁇ m average grain size and ⁇ 250 Hv hardness, and a composition consisting of, by mass, 0.08 to 0.20% C, 0.001 to ⁇ 0.2% Si, >1.0 to 3.0% Mn, 0.001 to 0.5% Al, >0.1 to 0.5% V, 0.05 to ⁇ 0.20% Ti, 0.005 to 0.05% Nb and the balance Fe with impurities and satisfying inequality (1) (Ti/48+Nb/93) ⁇ C/12 ⁇ 4.5 ⁇ 10 ⁇ 5 , inequality (2) 0.5 ⁇ (V/51+Ti/48+Nb/93)/(C/12) ⁇ 1.5 and inequality (3) V+Ti ⁇ 2+Nb ⁇ 1.4+C ⁇ 2 +Mn ⁇ 0.1 ⁇ 0.80, wherein the atomic symbols represent the respective contents (unit: Mass %) of
• JP-B 3821036 proposes a technique regarding a hot rolled steel sheet, characterized in that: the hot rolled steel sheet has a composition containing by mass %, 0.0002 to 0.25% C, 0.003 to 3.0% Si, 0.003 to 3.0% Mn and 0.002 to 2.0% Al, and balance as Fe and incidental impurities, wherein P, S and N contents in the incidental impurities are 0.15% or less, 0.05% or less and 0.01% or less, respectively; at least 70%, by area ratio, of metal microstructure is ferrite phase; the average crystal grain size of a ferrite phase is ⁇ 20 ⁇ m; the aspect ratio of the ferrite phase is ⁇ 3; ⁇ 70% of the ferrite grain boundaries consist of large-angled grain boundaries; the area ratio of precipit
• JP-A 2009-052139 proposes a technique regarding a high-strength steel sheet excellent in stretch-flange ability after forming and corrosion resistance after coating, comprising: a composition containing by mass %, C: 0.02 to 0.20%, Si: 0.3% or below, Mn: 0.5 to 2.5%, P: 0.06% or below, S: 0.01% or below, Al: 0.1% or below, Ti: 0.05 to 0.25%, and V: 0.05 to 0.25% with the balance consisting of Fe and incidental impurities; and microstructure substantially constituted of ferrite single-phase, wherein contents of Ti, V, and solute V in precipitates of less than 20 nm in size in the ferrite single phase microstructure are 200 to 1,750 mass ppm, 150 to 1,750 mass ppm, and 200 to less than 1,750 mass ppm, respectively.
• JP-A 2009-052139 attempts to increase strength of a steel sheet by making precipitates contained in the steel sheet minute (less than 20 nm in size). Further, the technique described in JP-A 2009-052139 attempts to improve stretch-flange ability after forming process by using Ti-V containing precipitates as precipitates which can remain minute in a steel sheet and setting content of solute V contained in the steel sheet to be a desired range.
• JP-A 2009-052139 states that a high strength hot rolled steel sheet having tensile strength of at least 780 MPa and excellent in stretch-flange ability after forming and corrosion resistance after coating can be obtained according to the technique thereof.
• JP-A 2009-052139 also states that the hot rolled steel sheet thus obtained is suitable for a base sheet of a galvanized steel sheet having hot-dip galvanized coating or galvannealed coating formed thereon.
• JP-A 2009-052139 states that a hot rolled steel sheet having strength of 780 MPa class and excellent formability (elongation and stretch-flange ability) can be manufactured by the technique it proposes.
• the technique described in JP-A 2009-052139 specifies precipitate size to be ⁇ 20 nm and simply setting precipitate size to be “less than 20 nm or so” results in unstable precipitation strengthening capacity because fine precipitates each having particle diameter of less than 10 nm or so actually plays the main role in precipitation strengthening as revealed in JP-B 3591502.
• the technique proposed by JP-A 2009-052139 therefore causes a problem that it is difficult to reliably ensure strength equal to or higher than 980 MPa with maintaining excellent formability.
• JP-A 2009-052139 that attempt to obtain strength of at least 980 MPa in particular tends to make uniformity of steel sheet properties insufficient and cause variation in the properties (e.g. strength) in the steel sheet widthwise direction in particular, thereby making it impossible to attain satisfactory properties at end portions in the widthwise direction of a steel sheet.
• JP-A 2009-052139 causes a problem in that it is difficult to stably and reliably supply hot rolled steel sheets each having strength of at least 980 MPa when mass production of such hot rolled steel sheets on an industrial scale is essential in order to stably supply the steel sheets as a material of automobile parts to be mass-produced. Yet further, there arises another problem in JP-A 2009-052139 that production yield deteriorates due to possible failure in obtaining satisfactory properties at end portions in the widthwise direction of a steel sheet.
• JP-A 2009-052139 fails to study any specific means for obtaining a high tensile strength galvanized steel sheet having: uniform properties across the entire steel sheet in the widthwise direction thereof; desired mechanical properties (strength, elongation and stretch-flange ability); and good surface quality after formation of hot-dip galvanized coating (or galvannealed coating). That is, how to specifically obtain such a high tensile strength galvanized steel sheet as described above remains as an unsolved task in JP-A 2009-052139.
• the present invention aims at advantageously solving the prior art problems described above and an object thereof is to provide a high tensile strength galvanized steel sheet suitable for use in automobile parts and a manufacturing method thereof, which high tensile strength galvanized steel sheet has: tensile strength (TS) of at least 980 MPa; excellent formability (elongation, strength-flange ability and optionally bending properties) which makes the steel sheet applicable to both a material of a chassis member or the like to be press-formed to have complicated sectional configurations and a material of a skeleton member of an automobile; and good surface quality.
• TS tensile strength
• excellent formability elongation, strength-flange ability and optionally bending properties
• TS tensile strength
• Ti-V based carbide is effective as fine carbide contributing to precipitation strengthening, in view of reliably obtaining high strength.
• Ti content is equal to or higher than a predetermined content determined according to contents of N and S in the steel (i.e. Ti ⁇ 0.10+(N/14*48+S/32*48) and that contents of C, Ti and V in the steel as a material satisfy a predetermined relationship (0.8 ⁇ (Ti/48+V/51)/(C/12) ⁇ 1.2), in order to achieve stable precipitation of Ti-V based fine carbides.
• Formation of an internal oxide layer in a hot rolled steel sheet is suppressed and galvanizing properties of the steel sheet improves by setting coiling temperature in coiling of the hot rolled steel sheet to be lower than the coiling temperature suitable for precipitation of Ti-V based fine carbides.
• Such Ti-V based fine carbides precipitated during continuous annealing process as described above exhibits precipitation morphology in which the carbides are dispersion-precipitated within ferrite phase.
• Bending properties improve by setting the total content of solute Ti and solute V in steel to be equal to or higher than a predetermined content, in addition to carrying out the aforementioned settings or adjustments.
• the total content of solute Ti and solute V in steel can be increased to be equal to or higher than the predetermined content by controlling the cooling rate after finish rolling in hot rolling.
• the present invention has been contrived based on the aforementioned discoveries and primary features thereof are as follows.
• a high tensile strength galvanized steel sheet having tensile strength of at least 980 MPa and excellent formability comprising: a hot rolled steel sheet having (i) a composition including by mass %, C: 0.07% to 0.13% (inclusive of 0.07% and 0.13%), Si: 0.3% or less, Mn: 0.5% to 2.0% (inclusive of 0.5% and 2.0%), P: 0.025% or less, S: 0.005% or less, N: 0.0060% or less, Al: 0.06% or less, Ti: 0.10% to 0.14% (inclusive of 0.10% and 0.14%), V: 0.15% to 0.30% (inclusive of 0.15% and 0.30%), Solute V: 0.04% to 0.1% (inclusive of 0.04% and 0.1%), Solute Ti: 0.05% or less, and remainder as Fe and incidental impurities, (ii) microstructure with fine carbides dispersion precipitated therein, the fine carbides containing Ti and V and having the average particle diameter of less than 10
• composition of the hot rolled steel sheet further includes by mass % at least one type of elements selected from Cr: 1% or less and B: 0.003% or less.
• composition of the hot rolled steel sheet further includes by mass % at least one type of elements selected from Nb and Mo such that the total content thereof is equal to or lower than 0.01 mass %.
• a method for manufacturing a high tensile strength galvanized steel sheet having excellent formability comprising preparing a steel material, subjecting the steel material to hot rolling including rough rolling and finish rolling, cooling after completion of the finish rolling, and coiling to obtain a hot rolled steel sheet, and subjecting the hot rolled steel sheet to continuous annealing and one of hot-dip galvanizing and galvannealing in this order to obtain a galvanized steel sheet, the method is characterized in that it further comprises: preparing the steel material to have a composition including by mass %, C: 0.07% to 0.13% (inclusive of 0.07% and 0.13%), Si: 0.3% or less, Mn: 0.5% to 2.0% (inclusive of 0.5% and 2.0%), P: 0.025% or less, S: 0.005% or less, N: 0.0060% or less, Al: 0.06% or less, Ti: 0.10% to 0.14% (inclusive of 0.10% and 0.14%), V: 0.15% to 0.30% (inclusive of 0.15%
• composition of the steel material further includes by mass % at least one type of elements selected from Cr: 1% or less and B: 0.003% or less.
• TS tensile strength
• the galvanized steel sheet of the present invention is a steel sheet constituted of: a hot rolled steel sheet having (a) microstructure with fine carbides dispersion precipitated therein, the fine carbides containing Ti and V and having the average particle diameter of less than 10 nm, as well as volume ratio with respect to the entire microstructure of at least 0.007 and (b) matrix as ferrite phase having area ratio with respect to the entire microstructure of at least 97%; and hot-dip galvanized coating or galvannealed coating formed on a surface the hot rolled steel sheet.
• Ferrite phase at least 97% by area ratio with respect to the entire microstructure
• ferrite phase is essential in terms of ensuring good formability (elongation and stretch-flange ability) of a galvanized steel sheet in the present invention.
• Constituting microstructure of a galvanized steel sheet predominantly of ferrite phase having relatively low dislocation density and thus excellent ductility effectively improves elongation and stretch-flange ability of the galvanized steel sheet.
• Constituting microstructure of a galvanized steel sheet of ferrite single phase is preferable in terms of improving stretch-flange ability in particular.
• microstructure of a galvanized steel sheet does not need to be fully constituted of ferrite single phase and the good effect of ferrite phase described above is sufficiently demonstrated when the microstructure is substantially constituted of ferrite single phase, i.e. area ratio of ferrite phase with respect to the entire microstructure is at least 97%. Accordingly, area ratio of ferrite phase with respect to the entire microstructure is to be at least 97%.
• microstructural components other than ferrite phase include cementite, pearlite phase, bainite phase, martensite phase, retaind austenite phase and the like in the galvanized steel sheet of the present invention. Presence of these microstructural components other than ferrite phase is tolerated unless the total area ratio thereof with respect to the entire microstructure exceeds 3% or so.
• Carbides containing Ti and V tend to be fine carbides having extremely small average particle diameter.
• the present invention aiming at increasing strength of a galvanized steel sheet through dispersion precipitation of fine carbides in the galvanized steel sheet, thus utilizes fine carbides containing Ti and V as fine carbides to be dispersion-precipitated in a galvanized steel sheet.
• Titanium carbide not containing vanadium has been normally used when strength of a steel sheet is to be increased in the prior art.
• the present invention characteristically employs carbides containing both Ti and V. Titanium exhibits strong tendency to form carbides. Ti carbide not containing V therefore tends to be coarsened and makes less contribution to increasing strength of a steel sheet than Ti carbide containing V, eventually necessitating adding a larger amount of Ti and forming a larger amount of Ti carbide to impart the steel sheet with desired strength (tensile strength: 980 MPa).
• carbides in a steel material must be melted prior to hot rolling as described below when a hot rolled steel sheet of the galvanized steel sheet of the present invention is manufactured.
• melting all of titanium carbide necessitated to ensure desired strength (tensile strength: 980 MPa) of a galvanized steel sheet requires very high slab heating temperature prior to hot rolling (equal to or higher than 1300° C.) in a case where the galvanized steel sheet is to be imparted with the desired strength solely by titanium carbide.
• Such high slab heating temperature as described above significantly exceeds normal slab heating temperature prior to hot rolling and requires special facilities, thereby making it difficult to carry out the production by using already existing manufacturing facilities.
• the present invention employs composite carbide containing Ti and V as carbide to be dispersion-precipitated. Vanadium effectively suppresses coarsening of carbide because vanadium has less tendency to form carbide than titanium.
• composite carbide containing Ti and V significantly drops melting temperature of carbide, as compared with a case using carbide containing Ti only, because combining both Ti with V very effectively drops melting temperature of carbide. That is, use of composite carbide containing Ti and V as carbide to be dispersion-precipitated is very advantageous in terms of production efficiency because the carbide melts at normal slab heating temperature prior to hot rolling even in a case where a large amount of carbide is to be dispersion precipitated for the purpose of imparting a galvanized steel sheet with desired strength (tensile strength: at least 980 MPa).
• “Fine carbides containing Ti and V” do not mean mixture of Ti carbides and V carbides respectively contained in microstructure but represent composite carbides each containing both Ti and V within one fine carbide particle.
• Average particle diameter of fine particle less than 10 nm
• the average particle diameter of fine carbides is very important in terms of imparting a galvanized steel sheet with desired strength (tensile strength: at least 980 Mpa).
• the average particle diameter of fine carbides containing Ti and V is to be less than 10 nm in the present invention.
• Fine carbides precipitated in matrix of a galvanized steel sheet function as resistance against dislocation motion occurring when a steel sheet is deformed, thereby increasing strength of the galvanized steel sheet, and this strength-increasing effect of fine carbides is conspicuous when the average particle diameter of the fine carbides is less than 10 nm. Accordingly, the average particle diameter of fine carbides containing Ti and V is to be less than 10 nm and preferably 5 nm or less.
• volume ratio of fine carbides with respect to the entire microstructure at least 0.007
• a dispersion-precipitated state of fine carbides containing Ti and V is also very important in terms of imparting a galvanized steel sheet with desired strength (tensile strength: at least 980 Mpa).
• Fine carbides containing Ti and V and having the average particle diameter of less than 10 nm are dispersion-precipitated such that fraction in microstructural terms of the fine carbides with respect to the entire microstructure is at least 0.007 in the present invention. In a case where this fraction is less than 0.007, it is difficult to reliably obtain desired strength (tensile strength: at least 980 MPa) of a galvanized steel sheet, although the average particle diameter of fine carbides containing Ti and V is less than 10 nm in the galvanized steel sheet. Accordingly, the fraction is to be at least 0.007 and preferably at least 0.008.
• Precipitation morphology of fine carbides containing Ti and V in the present invention includes a state in which randomly-precipitated fine carbides exist in a mixed manner, as well as a main precipitation state in which fine carbides are precipitated in row.
• the former randomly-precipitated state causes no adverse effect on the properties of a galvanized steel sheet. Morphology of precipitation therefore does not matter and various types of precipitation states may be collectively referred to as “dispersion precipitation” in the present invention.
• Carbon is an essential element in terms of forming fine carbides and increasing strength of a galvanized steel sheet. Carbon content in steel less than 0.07% makes it impossible to reliably obtain fine carbides at desired microstructural fraction in a resulting galvanized steel sheet, whereby the steel sheet cannot have tensile strength of at least 980 MPa. However, carbon content in steel exceeding 0.13% causes troubles such as difficulty in spot welding. Accordingly, carbon content in steel is to be in the range of 0.07% to 0.13% (inclusive of 0.07% and 0.13%) and preferably in the range of 0.08% to 0.12% (inclusive of 0.08% and 0.12%).
• Silicon content in steel exceeding 0.3% facilitates precipitation of carbon from ferrite phase and precipitation of coarse Fe carbide at grain boundaries, thereby deteriorating stretch-flnageability of a resulting galvanized steel sheet. Further, Si content in steel exceeding 0.3% increases rolling road to render shape of a rolled material unsatisfactory. Accordingly, Si content in steel is to be 0.3% or less, preferably 0.15% or less, and more preferably 0.05% or less.
• Mn 0.5% to 2.0% (inclusive of 0.5% and 2.0%)
• Manganese is a solute strengthening element and effectively increases strength of a steel sheet.
• Manganese content in steel is preferably at least 0.5% in terms of increasing strength of a galvanized steel sheet.
• Mn content in steel exceeding 2.0% results in apparent manganese segregation and formation of a phase other than ferrite phase, i.e. formation of a hard phase, thereby deteriorating stretch-flange ability of a resulting galvanized steel sheet.
• Mn content in steel is to be in the range of 0.5% to 2.0% (inclusive of 0.5% and 2.0%) and preferably in the range of 1.0% to 2.0% (inclusive of 1.0% and 2.0%).
• Phosphorus content in steel exceeding 0.025% results in apparent phosphorus segregation and deteriorate stretch-flange ability of a resulting galvanized steel sheet.
• phosphorus content in steel is to be 0.025% or less and preferably 0.02% or less.
• S 0.005% or less
• P Sulfur is an element which deteriorates hot formability (hot rolling formability), makes a slab susceptible to hot cracking, and forms MnS in steel to deteriorate formability (stretch-flange ability) of a hot rolled steel sheet. Accordingly, sulfur content in steel is preferably reduced as best as possible in the present invention. Sulfur content in steel is to be 0.005% or less and preferably 0.003% or less.
• Nitrogen is a harmful element and content thereof in steel is preferably reduced as best as possible in the present invention. Nitrogen content exceeding 0.0060% results in formation of coarse nitride in steel, and the coarse nitride eventually deteriorates stretch-flange ability. Accordingly, nitrogen content in steel is to be 0.0060% or less.
• Aluminum is an element which functions as a deoxidizing agent.
• Aluminum content in steel is preferably at least 0.001% to sufficiently obtain the deoxidizing effect of aluminum.
• Al content in steel exceeding 0.06% deteriorates elongation and stretch-flange ability of a resulting galvanized steel sheet. Accordingly, aluminum content in steel is to be 0.06% or less.
• Titanium is one of the important elements in the present invention. Titanium is an element which forms composite carbide with vanadium to contribute to increasing strength of a steel sheet with maintaining excellent elongation and stretch-flange ability thereof. Titanium content in steel less than 0.10% cannot ensure desired strength (tensile strength: at least 980 MPa) of a galvanized steel sheet. However, Ti content in steel exceeding 0.14% deteriorates stretch-flange ability of a galvanized steel sheet. Further, Ti content in steel exceeding 0.14% possibly results in a situation in which carbides fail to melt unless slab heating temperature prior to hot rolling is raised to 1300° C. or higher when a hot rolled steel sheet as a base sheet of a galvanized steel sheet is manufactured.
• Titanium content in steel is therefore to be in the range of 0.10% to 0.14% (inclusive of 0.10% and 0.14%).
• V 0.15% to 0.30% (inclusive of 0.15% and 0.30%)
• Vanadium is one of the important elements in the present invention. Vanadium is an element which forms composite carbide with titanium to contribute to increasing strength of a steel sheet with maintaining excellent elongation and stretch-flange ability thereof. Vanadium content in steel less than 0.15% cannot ensure desired strength (tensile strength: at least 980 MPa) of a steel sheet. However, V content in steel exceeding 0.30% makes center segregation thereof apparent, thereby deteriorating elongation and/or toughness of a resulting galvanized steel sheet. Accordingly, vanadium content in steel is to be in the range of 0.15% to 0.30% (inclusive of 0.15% and 0.30%).
• contents of C, N, S, Ti and V are controllably set so as to satisfy the aforementioned ranges and formula (1) and formula (2) below, respectively.
• Fine carbides containing Ti and V are dispersion-precipitated in a galvanized steel sheet in the present invention, as described above. These fine carbides, in a steel material, are melted when the steel material is heated prior to hot rolling and then precipitated during subsequent hot rolling, cooling after the hot rolling, coiling and continuous annealing. The fine carbides are formed such that Ti is first precipitated as nucleus and then V is precipitated to form a composite therewith.
• Contents of Ti, N and S in steel are therefore to be controllably set to satisfy formula (1), i.e. Ti ⁇ 0.10+(N/14*48+S/32*48).
• Setting contents of Ti, N and S in steel to satisfy formula (1) ensures sufficient content of Ti as precipitation nuclei of fine carbides, makes the fine carbides be stably precipitated as fine carbides having the average particle diameter of 10 nm or less, and thus realizes dispersion precipitation in which volume ratio of the fine carbides with respect to the entire microstructure of an eventually obtained galvanized steel sheet is at least 0.007.
• contents of Ti, N and S in steel as a material of the galvanized steel sheet are controllably set to satisfy formula (1), i.e. Ti ⁇ 0.10+(N/14*48+S/32*48), in the present invention.
• contents of Ti, (V and C) contents in steel as a material of the galvanized steel sheet are controllably set to satisfy formula (2), i.e. 0.8 ⁇ (Ti/48+V/51)/(C/12) ⁇ 1.2, in the present invention.
• Solute V 0.04% to 0.1% (inclusive of 0.04% and 0.1%)
• Solute vanadium effectively functions to improve stretch-flange ability of a galvanized steel sheet.
• content of solute V among vanadium contained in a galvanized steel sheet is less than 0.04%, the aforementioned good effect of vanadium is not sufficiently demonstrated and a resulting galvanized steel sheet cannot reliably have stretch-flange ability good enough for application to a material of a chassis member or the like to be formed to have complicated cross-sectional configurations.
• the content of solute V exceeding 0.1% not only the good effect of vanadium reaches a plateau but also fine carbides containing Ti and V necessitated to ensure desired strength (tensile strength: at least 980 MPa) of a steel sheet may not be sufficiently obtained.
• content of solute V among vanadium contained in a galvanized steel sheet is to be in the range of 0.04% to 0.1% (inclusive of 0.04% and 0.1%), preferably in the range of 0.04% to 0.07% (inclusive of 0.04% and 0.07%), and more preferably in the range of 0.04% to 0.06% (inclusive of 0.04% and 0.06%).
• the galvanized steel sheet of the present invention contains solute V by desired content in order to ensure good stretch-flange ability of the galvanized steel sheet as described above.
• Solute titanium does not cause such a good effect as solute V does and presence of solute Ti rather means that content of Ti effectively functioning as precipitation nucleus has been decreased accordingly.
• Content of solute Ti is therefore to be 0.05% or less, preferably 0.03% or less, and more preferably 0.02% or less to ensure desired strength (tensile strength: at least 980 MPa) of a resulting steel sheet.
• Grain boundaries of steel is strengthened and bending properties of a resulting steel sheet improves by setting the total content of solute V and solute Ti present in ferrite phase to be at least 0.07%.
• contents of solute V and solute Ti it is preferable to set contents of solute V and solute Ti to be in the aforementioned corresponding ranges, respectively, and adjust the total content of solute V and solute Ti to at least 0.07%.
• the total content of solute V and solute Ti is lower than 0.07%, the desired effect of strengthening grain boundaries and improving bending properties described above cannot be obtained.
• fine carbides containing Ti and V may not be sufficiently precipitated.
• the total content of solute V (0.04% to 0.1%, inclusive of 0.04% and 0.1%) and solute Ti (0.04% or less) is to be 0.15% or less.
• the total content of solute V and solute Ti is preferably 0.10% or less in terms of effectively utilizing V and Ti contained in a steel sheet.
• composition of the hot rolled steel sheet as a base sheet of the galvanized steel sheet of the present invention may contain, in addition to the basic components described above, at least one type of element selected from Cr: 1% or less and B: 0.003% or less.
• Chromium and boron are elements each functioning to increase strength of steel and may be selected and included in the composition according to necessity.
• Chromium is an element which in solute state effectively strengthens ferrite phase. Chromium content in steel is preferably at least 0.05% in order to obtain such a good effect of chromium as described above. However, Cr content in steel exceeding 1% is not economical because the good effect of Cr then reaches a plateau. Accordingly, Cr content in steel is preferably 1% or less.
• Boron is an element which effectively lowers the Ar a transformation point of steel and may be utilized to adjust area ratio of ferrite phase with respect to the entire microstructure during cooling process in hot rolling.
• boron content in steel is preferably 0.003% or less.
• Content of boron, in a case where it is utilized, is preferably at least 0.0005% to reliably obtain the good effect thereof.
• the composition of the hot rolled steel sheet as a base sheet of the galvanized steel sheet of the present invention may contain by mass %, in addition to the basic components described above, at least one type of element selected from Nb and Mo such that the total content thereof is equal to or lower than 0.01 mass %.
• the composition may include Nb and Mo according to necessity because Nb and Mo are compositely precipitated with Ti and V to form composite carbide, thereby contributing to obtaining desired strength of a steel sheet.
• the total content of Nb and Mo is preferably at least 0.005% in order to sufficiently obtain the good effect of Nb and Mo.
• too high total content of Nb and Mo tends to deteriorate elongation of a resulting steel sheet. Accordingly, the composition preferably contains at least one of Nb and Mo such that the total content thereof is 0.01% or less.
• Components other than those described above are Fe and incidental impurities in the galvanized steel sheet of the present invention.
• the incidental impurities include O, Cu, Sn, Ni, Ca, Co, As and the like. Presence of these impurities is tolerated unless contents thereof exceed 0.1%. Contents of these impurities are preferably 0.03% or less.
• the method basically includes preparing a steel material, subjecting the steel material to hot rolling including rough rolling and finish rolling, cooling after completion of the finish rolling, and coiling to obtain a hot rolled steel sheet, and subjecting the hot rolled steel sheet to continuous annealing and one of hot-dip galvanizing and galvannealing in this order to obtain a galvanized steel sheet.
• the method preferably further includes:
• the method preferably yet further includes setting the average cooling rate in the cooling process after the hot roling to be at least 20° C/s.
• the smelting technique for preparing a steel material is not particularly restricted and any of the known smelting techniques such as a converter, an electric furnace or the like can be employed in the present invention.
• a slab (the steel material) is preferably prepared by continuous casting after smelting process in view of problems such as possible segregation, although a slab may be prepared by a known casting method such as ingot casting-rolling (blooming), thin slab continuous casting or the like.
• a cast slab is hot rolled, the slab may be either rolled after being reheated by a heating furnace or immediately rolled without being reheated when the temperature of the slab is kept at predetermined temperature or higher.
• the steel material thus obtained is then subjected to rough rolling and finish rolling.
• Carbides contained in the steel material must be melted prior to rough rolling in the present invention.
• the steel material is heated in this regard to temperature preferably in the range of 1150° C. to 1280° C. (inclusive of 1150° C. and 1280° C.) because the steel material of the present invention contains Ti and V as carbide-forming elements.
• This process of heating a steel material prior to rough rolling may be omitted in a case where the steel material prior to rough rolling is kept at temperature equal to or higher than predetermined temperature and carbides in the steel material have been melted as described above. Conditions of rough rolling need not be particularly restricted.
• Finish rolling completing temperature 880° C. or higher
• Adequately setting finish rolling completing temperature is important in terms of ensuring good elongation and stretch-flange ability of a hot rolled steel sheet and decreasing rolling load in finish rolling.
• Finish rolling completing temperature lower than 880° C. results in coarse crystal grains at surface layers of a hot rolled steel sheet, which deteriorate elongation and stretch-flange ability of the steel sheet.
• finish rolling completing temperature is lower than 880° C.
• magnitude of accumulated strains introduced into a rolled material increases because rolling is carried out in non-recrystallization temperature region; and rolling load significantly increases as the magnitude of accumulated strains increases, thereby making it difficult to reduce thickness of a hot rolled steel sheet.
• finish rolling completing temperature is to be 880° C. or higher and preferably 900° C. or higher.
• finish rolling completing temperature is preferably 1000° C. or lower because too high finish rolling completing temperature coarsens crystal grains of a steel sheet to cause an adverse effect on obtaining desired strength (tensile strength : at least 980 MPa) in the steel sheet.
• Coiling temperature 480° C. to 580° C. (inclusive of 480° C. and exclusive of 580° C.
• Adequately setting coiling temperature in the coiling process is very important in terms of suppressing formation of an internal oxide layer in a hot rolled steel sheet (a hot rolled sheet) as a base sheet of the galvanized steel sheet and obtaining desired microstructure across the entire steel sheet in the widthwise direction in the eventually obtained galvanized steel sheet, which desired microstructure includes: fine carbides dispersion-precipitated therein, the fine carbides containing Ti and V and having the average particle diameter of less than 10 nm, as well as volume ratio with respect to the entire microstructure of at least 0.007; and matrix as ferrite phase having area ratio with respect to the entire microstructure of at least 97%.
• Coiling temperature lower than 480° C. causes fine carbides to be insufficiently precipitated at end portions in the widthwise direction of a rolled material, which portions are susceptible to excessive cooling, thereby making it impossible to impart the eventually obtained galvanized steel sheet with desired tensile strength; induces formation of a hard secondary phase, thereby deteriorating elongation of the eventually obtained galvanized steel sheet; and problematically deteriorates running stability on a run-out table.
• Coiling temperature equal to or higher than 580° C. causes an internal oxide layer to be significantly formed in a hot rolled steel sheet (a hot rolled sheet) as a base sheet of the galvanized steel sheet, thereby deteriorating coatability of the galvanized steel sheet.
• coiling temperature is to be in the range of 480° C. to 580° C. (inclusive of 480° C. and exclusive of 580° C.). “Coiling temperature” represents coiling temperature actually measured at the center portion in the widthwise direction of a rolled material or coiling temperature at the center portion in the widthwise direction of the rolled material calculated through simulation or the like in the present invention.
• Cooling after completion of finish rolling down to the coiling temperature is preferably carried out at the average cooling rate of at least 20° C./s.
• the average cooling rate in cooling after completion of finish rolling from temperature equal to or higher than 880° C. down to the coiling temperature is lower than 20° C./s, the Ar 3 transformation point tends to be high and carbides containing Ti and V are likely to be coarsened, whereby consumption of solute V and solute Ti in steel is facilitated, which solute V and solute Ti would otherwise effectively improve bending properties of a resulting steel sheet.
• the lower limit of the average cooling rate is preferably 60° C/s in terms of preventing uneven cooling from occurring, although the upper limit if not particularly restricted.
• the hot rolled steel sheet thus obtained is then subjected to continuous annealing, galvanizing and optionally alloying in this order to produce a galvanized steel sheet in the present invention.
• Adequately setting the annealing temperature is critically important here. It is preferable to carry out these continuous annealing, galvanizing and optionally alloying processes in a continuous galvanizing line (CGL) in terms of production efficiency.
• Annealing temperature 750° C. or lower
• the coiling temperature during coiling of a hot rolled steel sheet is set to be relatively low in order to suppress formation of an internal oxide layer in the hot rolled steel sheet as a base sheet of a galvanized steel sheet in the present invention, as described above. That is, the coiling temperature of a hot rolled steel sheet in the present invention is set to be lower than the coiling temperature which would be suitable for precipitation of fine carbides containing Ti and V. As a result, precipitation of fine carbides containing Ti and V tends to be insufficient at end portions in the widthwise direction of the hot rolled steel sheet.
• the annealing temperature in continuous annealing process is optimized to facilitate precipitation of fine carbides containing Ti and V during the continuous annealing process in the present invention.
• the annealing temperature which is adequate in terms of reliable dispersion precipitation of fine carbides containing Ti and V and achieving the average particle diameter of 10 nm or less of such fine carbides at volume ratio thereof with respect to the entire microstructure of at least 0.007 in an eventually obtained galvanized steel sheet, is 750° C. or lower.
• the annealing temperature need not exceed 750° C. because then the effect of facilitating precipitation of fine carbides reaches a plateau.
• the annealing temperature is preferably 700° C. or lower.
• the annealing temperature is preferably equal to or higher than 600° C. because annealing temperature lower than 600° C. may result in insufficient precipitation of the fine carbides.
• the hot rolled steel sheet is preferably retained at the aforementioned annealing temperature for a period ranging from 120s to 300s (inclusive of 120s and exclusive of 300s), although conditions other than the annealing temperature are not particularly restricted in the continuous annealing process.
• Conditions of the galvanizing process and the alloying process are not particularly restricted, either, and hot-dip galvanized coating or galvannealed coating can be formed under the known conditions generally associated therewith.
• TS tensile strength
• excellent formability elongation and stretch-flange ability
• composition of a steel material of a galvanized steel sheet is controlled in the present invention such that Ti content is equal to or higher than a predetermined content determined according to contents of N and S in the steel material (Ti ⁇ 0.10+(N/14*48+S/32*48) and that contents of C, Ti and V in the steel material satisfy a predetermined relationship (0.8 ⁇ (Ti/48+V/51)/(C/12) ⁇ 1.2), so that fine carbides having the average particle diameter of 10 nm are dispersion-precipitated sufficiently.
• the coiling temperature of a hot rolled steel sheet in the present invention is set to be lower than the coiling temperature suitable for precipitation of fine carbides containing Ti and V, whereby precipitation of fine carbides containing Ti and V tends to be insufficient at end portions in the widthwise direction in particular of the hot rolled steel sheet.
• fine carbides containing Ti and V are made to be precipitated during the continuous annealing process prior to the galvanizing process in the present invention, by controlling the composition of the steel material to satisfy formula (1) and (2) above.
• fine carbides having the average particle diameter of 10 nm or less can be precipitated at end portions in the widthwise direction of the hot rolled steel sheet by the continuous annealing process, in spite of possible excessive cooling at the end portions in the widthwise direction of the steel sheet in cooling and coiling after completion of finish rolling, whereby fine carbides having the average particle diameter of 10 nm or less are precipitated at the desired volume ratio (at least 0.007) across the entire steel sheet in the widthwise direction thereof and satisfactory properties (tensile strength, elongation, stretch-flange ability) are ensured in an eventually obtained galvanized steel sheet in the present invention.
• the total content of solute V and solute Ti can be increased to 0.07% or higher in the hot rolled steel sheet subjected to galvanizing, by setting the average cooling rate from temperature 880° C. or lower after completion of finish rolling down to the coiling temperature to be at least 20° C./s. As a result, a galvanized steel sheet having good bending properties can be obtained.
• a galvanized steel sheet having excellent surface quality can be obtained in the present invention because formation of an internal oxide layer is suppressed in a hot rolled steel sheet of the galvanized steel sheet.
• Each of molten steel samples having respective compositions shown in Table 1 was subjected to smelting and continuous casting by the conventional known techniques to obtain a slab (a steel material) having 250 mm thickness.
• the slab was subjected to heating at 1250° C., rough rolling, finish rolling at the corresponding finish rolling completing temperature shown in Table 2, and coiling at the corresponding coiling temperature shown in Table 2, whereby a hot rolled steel sheet sample having sheet thickness: 2.3 mm was obtained.
• Each of the hot rolled steel sheet samples thus obtained was subjected to: continuous annealing in a continuous galvanizing line under the conditions including the corresponding annealing temperature and the retention time at the annealing temperature shown in Table 2; and immersion in molten zinc at 550° C. for galvanizing to form hot-dip galvanized coating on surfaces of the hot rolled steel sheet sample, whereby a galvanized steel sheet sample was manufactured.
• Coating weight was 50 g/m 2 for each galvanized steel sheet sample.
• Example F 0.096 0.02 1.54 0.010 0.0024 0.0076 0.042 0.104 0.267 — 0.130 0.925
• Example G 0.088 0.01 1.13 0.010 0.0007 0.0033 0.040 0.091 0.271 — 0.112 0.983
• Test pieces were collected from each of the galvanized steel sheet samples thus obtained. These test pieces were subjected to microstructural observation, a tensile test and a hole-expansion test, whereby area ratio of ferrite phase, the average particle diameter and volume ratio of fine carbides containing Ti and V, content of solute V, content of solute Ti, presence/absence of internal oxide layer, tensile strength, total elongation, and hole expansion ratio (stretch-flange ability) were determined. Testing methods were as follows.
• a test piece was collected from the center portion in the sheet widthwise direction of the hot rolled steel sheet portion of each of the galvanized steel sheet samples thus obtained (the hot rolled steel sheet portion was a portion other than hot-dip galvanized coating, of the galvanized steel sheet sample).
• SEM scanning electron microscope
• the photograph of microstructure was analyzed by using an image analyzer to identify ferrite phase and the phases other than ferrite phase and determine respective area ratios of these phases.
• a thin film was prepared from the center portion in the sheet widthwise direction of the hot rolled steel sheet portion of each of the galvanized steel sheet samples (the hot rolled steel sheet portion was a portion other than hot-dip galvanized coating, of the galvanized steel sheet sample).
• the thin film was observed by using a transmission electron microscope (TEM) to determine particle diameters and volume ratio of fine carbides containing Ti and V.
• TEM transmission electron microscope
• content of solute Ti and content of solute V were determined by: treating a test piece of each galvanized steel sheet sample with 10% acetylacetone-1% tetramethylammonium-methanol solution as electrolytic solution to obtain extraction residue; chemically analyzing the extraction residue to determine Ti content and Vi content as precipitates, respectively; and subtracting the Ti content and the Vi content as precipitates thus determined from the total Ti content and the total V content, respectively, to determine content of solute Ti and content of V.
• vicinities of the surface layer of a test piece of each galvanized steel sheet was observed by using a scanning electron microscope (SEM) at magnification of 5000 x to determine presence/absence of an internal oxide layer.
• JIS Z 2201 A JIS No. 5 tensile test piece (JIS Z 2201), of which tensile direction coincided with the direction orthogonal to the rolling direction, was collected from each of the galvanized steel sheet samples thus obtained. Tensile tests were carried out by using the test piece according to JIS Z 2241 to determine tensile strength (TS) and total elongation (El) of the test piece.
• a test piece (size: 130mm*130mm) was collected from each of the galvanized steel sheet samples thus obtained and a hole (the initial diameter d 0 : 10mm ⁇ ) was formed by punching in the test piece.
• a hole expansion test was carried out by using the test piece thus punched by: inserting a cone-shaped punch having apex angle of 60° into the hole to expand the hole; measuring diameter d of the hole when a crack penetrated through the steel sheet (the test piece); and calculating hole expansion ratio ⁇ (%) according to formula below.
• Example B 4 0.018 0.072 98.1 5 0.0082 1035 16.8 43.9
• Example C 5 0.023 0.061 97.9 3 0.0092 1045 16.8 45.3
• Example 6 0.011 0.065 97.5 4 0.0088 1042 16.8 44.1
• Example 7 0.024 0.052 97.4 18 0.0081 963 17.5 50.7 Comp.
• Example D 8 0.029 0.073 98.1 3 0.0079 1012 17.1 46.9
• Example E 9 0.019 0.006 97.8 5 0.0058 964 16.3 41.9
• Example F 10 0.013 0.078 97.6 4 0.0054 905 17.8 51.4
• Example G 11 0.010 0.085 98.2 5 0.0063 942 18.1 44.7 Comp.
• Example *3 Area ratio with respect to the entire microstructure (%) *4: Fine carbide containing Ti and V, of which “Volume ratio” represents volume ratio with respect to the entire microstructure
• Example *5 “Hot rolled steel sheet” portion other than galvanized coating, of galvanized steel sheet
• JIS No.5 tensile test pieces were collected from vicinities of end portions in the sheet widthwise direction thereof (i.e. edge portions), as well as the aforementioned center portion in the sheet widthwise direction, in the same manner as described above for an additional tensile test.
• the results of comparing the tensile strength (TS) measured at the center portion in the sheet widthwise direction, with the tensile strength (TS) measured in the vicinity of an end portion (i.e. an edge portion) in the sheet widthwise direction, are shown for the relevant Examples in Table 4.
• the galvanized steel sheets of the present invention each exhibit sufficiently high tensile strength TS at both the center portion and the vicinity of an end portion (an edge portion) in the sheet widthwise direction thereof, i.e. demonstrate excellent properties at end portions in the sheet widthwise direction thereof, as well.
• Each of molten steel samples having respective compositions shown in Table 5 was subjected to smelting and continuous casting by the conventionally known techniques to obtain a slab (a steel material) having 250 mm thickness.
• the slab was subjected to heating at 1250° C., rough rolling, finish rolling at the corresponding finish rolling completing temperature shown in Table 6, cooling (from the finish rolling completing temperature down to the coiling temperature) at the corresponding average cooling rate shown in Table 6, and coiling at the corresponding coiling temperature shown in Table 6, whereby a hot rolled steel sheet sample having sheet thickness: 2.3 mm was obtained.
• Test pieces were collected from (the center portion in the sheet widthwise direction of) each of the galvanized steel sheet samples thus obtained. These test pieces were subjected to microstructural observation, a tensile test and a hole-expansion test, whereby area ratio of ferrite phase, the average particle diameter and volume ratio of fine carbides containing Ti and V, content of solute V, content of solute Ti, presence/absence of internal oxide layer, tensile strength, total elongation, and hole expansion ratio (stretch-flange ability) were determined. Testing methods were the same as those employed in Experiment 1.
• a bending test piece was collected from each of the galvanized steel sheet samples thus obtained.
• the bending test piece was subjected to a bending test. Testing conditions were as follows.
• Bending test pieces (30 mm*150 mm each) was collected from each of the galvanized steel sheet samples thus obtained such that the longitudinal direction of each test piece was oriented orthogonal to the rolling direction.
• the bending test pieces were subjected to a V-block bend test (bending angle:90°) according to JIS Z 2248. The test was carried out for three test pieces, respectively, by: measuring the smallest bending radius R (mm) at which generation of crack was narrowly avoided; dividing R by the sheet thickness t (mm); and regarding R/t as the limit bending radius.
• Examples according to the present invention of Experiment 2 unanimously realized galvanized steel sheets each having sufficiently high strength (tensile strength TS: at least 980 MPa) and excellent formability (total elongation E1: at least 15%, and hole expansion ratio ⁇ : at least 40%) with significantly suppressed formation of an internal oxide layer.
• Examples according to the present invention of Experiment 2 unanimously realized galvanized steel sheets each having excellent bending properties of limit bending radius R/t ⁇ 0.7, in addition to sufficiently high strength (tensile strength TS: at least 980 MPa) and excellent formability (total elongation E1: at least 15%, and hole expansion ratio ⁇ : at least 40%), when the total content of solute V and solute Ti is equal to or higher than 0.07%.
• JIS No.5 tensile test pieces were collected from vicinities of end portions in the sheet widthwise direction thereof (i.e. edge portions), as well as from the center portion in the sheet widthwise direction thereof, in the same manner as in Experiment 1 for additional tensile test.
• the results of comparing the tensile strength (TS) measured at the center portion in the sheet widthwise direction, with the tensile strength (TS) measured in the vicinity of an end portion (i.e. an edge portion) in the sheet widthwise direction, are shown for the relevant Examples in Table 8.
• the galvanized steel sheets of the present invention each exhibit sufficiently high tensile strength TS at both the center portion and the vicinity of an end portion (an edge portion) in the sheet widthwise direction thereof, i.e. demonstrate excellent properties at end portions in the sheet widthwise direction thereof, as well.

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• 2011
  • 2011-03-25 JP JP2011067418A patent/JP5041083B2/ja active Active
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  • 2011-03-30 EP EP11762272.0A patent/EP2554705B1/en active Active
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  • 2011-03-30 WO PCT/JP2011/001930 patent/WO2011122030A1/ja active Application Filing
  • 2011-03-31 TW TW100111282A patent/TWI460307B/zh active

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